1. Field of the Invention
The present invention relates to emission-gated electron sources and more particularly to an apparatus and method for significantly compressing current pulses in an electron source to enable applications such as frequency multipliers and terahertz frequency sources.
2. Description of Related Art
In a conventional emission-gated microwave vacuum tube amplifier, the electron beam is density modulated at the electron gun. The most common technique applies an input signal to a resonant cavity to develop peak RF fields between the cathode surface and a closely-spaced control grid. At favorable phases, the RF field enhances emission such that the electron beam is modulated at the drive frequency and amplitude. Beyond the control grid, the beam is accelerated by anode potential. An output cavity extracts the amplified RF current from the beam. Such an approach is generally used in the design of an inductive output tube (IOT), an example of which is illustrated in FIGS. 1 and 2.
Typically, the control grid of an IOT is biased to generate an idle current. At low drive levels, this allows for Class A modulation, by which the electron beam is modulated over the full RF cycle. At high drive levels, the RF voltage overwhelms the grid bias such that emission is suppressed during the negative half cycle, enabling Class B operation. If the grid bias is adjusted more negative, Class C operation can be achieved whereby the cathode emits over less than half of an RF cycle. This produces a shorter bunch of electrons, which reduces the phase variation, or transit angle, in the output gap, thereby improving electronic efficiency. However, Class C operation requires more drive power to produce a given beam current, thereby reducing gain. Class C operation is thus not typically used in conventional IOTs.
Nevertheless, for certain applications and in certain devices, reducing the pulse width is necessary. For example, in frequency multipliers, which are under increasing consideration as a means for generating terahertz radiation, the rate of phase variation in the output gap is multiplied by the same factor as the frequency, requiring proportional compression of the current pulse. At best, however, this technique reduces the pulse width by only fifty percent, sufficient only for frequency doublers. Accordingly, it is desirable to provide an apparatus and method for achieving far greater electron pulse compression factors in order to enable, for example, a frequency multiplier driven at X-band to produce output power at sub-millimeter wavelengths.